A structure of this kind is known from HENDERSON in Fr No. 2270390. The HENDERSON structure is erected offshore by installing the column and legs in step-by-step fashion onto a pre-installed base structure. A pivotal connection is made between the legs and the base structure to enable the legs to be swung into position relative to the column. The legs are then attached to the column by a grouted connection. One of the difficulties in the HENDERSON structure is the pivotal connection of the legs to the base structure. HENDERSON proposes for this a pin-type universal coupling. Bearing in mind the size of the components of the HENDERSON structure and the fact that the connection has to be made at the bottom of the ocean, the HENDERSON proposal is in this respect considered to be unrealistic in practice. Another difficulty is that in the HENDERSON structure the weight of the column is not fully supported by the legs. Even after the grouted connection has been made, some of the weight of the column in HENDERSON has to be borne by the base structure. The base structure must therefore be made sufficiently massive to take this weight. The structure developed by UITTENBOGAARD et al and shown in GB No. 2096673 offers neat solutions to these problems. UITTENPOGAARD concerns a structure which is of the same kind which is erected offshore by installing the column and legs in step-by-step fashion onto a pre-installed base structure. The UITTENBOGAARD base structure has upwardly facing wedge-shaped recesses which enables the pivotal connection of the legs to be made by simple lowering of the legs vertically into the recesses. At ocean depths of up to 450 meters, and with correspondingly sized components parts, this is consideraly easier to achieve than HENDERSON'S proposed connections. Furthermore, UITTENBOGAARD provides a nodal structure integrally on the column with downwardly extending projections for the legs. The column is lowered to a first depth and the legs are swung into position immediately below the column projections. Then the column is lowered further to a second depth so that the legs are received and located in the column projections. The connection is then made between the column and legs. By this, UITTENBOGAARD is able to ensure that the weight of the column is fully supported by the legs.
The present application is made by the same Assignee as that of UITTENBOGAARD et al. The development leading to the present application is primarily concerned with shallower water structures than that for which the UITTENBOGAARD structure was developed. Thus the present invention offers a useful alternative to the UITTENBOGAARD structure for installations where the water depth and conditions allow.
The present invention is based on the idea of fabricating an offshore structure in essentially only two pieces, a pre-formed base unit, which can be anchored on the sea bed, and a column. The base unit is provided with a sleeve through which the column can be slidingly received. The structure is erected by slidingly engaging the column with the sleeve of the base unit. This can be either in horizontal mode, parallel to the surface of the water, or in vertical mode, by lowering the column vertically with respect to the base unit. The column is attached to the base unit by means of a rigid connection between the column and sleeve. The sleeve is of sufficient dimensions that when the rigid connection is made between the column and sleeve, the sleeve is able to accept the primary load of the column, such load thus being transmitted through the legs and base structure of the base unit to the sea bed. A structure having the strength advantages of the UITTENBOGAARD structure is thus provided, but with the additional advantage that its fabrication and installation is greatly simplified.